Question

What volume of $0.10$M sodium formate solution should be added to $50$ml of $0.05$M formic acid to produce a buffer solution of $pH$ $4.0$?

[p$K_a$ for formic acid is $3.80$]

• A. $3.96$ml
• B. $25$ml
• C. $39.6$ml
• D. $100$ml

Answer 1

The correct option is C $39.6$ml

Let $V$ mL of $0.10M$ sodium formate solution should be added to $50ml$ of $0.05M$ formic acid to produce a buffer solution of $pH$ $\mathrm{4.0.}$

$pH=pKa+log\frac{\left[HCOONa\right]}{\left[HCOOH\right]}$

$4.0=3.80+log\frac{V\times 0.10}{50\times 0.05}$

$0.2=log0.04V$

$0.04V=1.585$

$V=39.6$ mL.

Hence, the correct option is $\text{C}$

Option C is correct.